This invention relates to internal reflection spectroscopy; and its primary purpose is to provide an apparatus which permits relatively deep immersion of an internal reflectance accessory into a container whose contents are to be analyzed. The analysis is accomplished by attenuated total reflectance (ATR), for which an internal reflectance element (IRE) is used. For the range of materials of primary interest, the desired analytical radiation is mid-infrared (MIR).
In an earlier application, U.S. Ser. No. 158,214, filed Feb. 19, 1988, now U.S. Pat. No. 4,835,389, the inventor in the present application disclosed several versions of such an apparatus. The present application has a different assignee from U.S. Pat. No. 4,835,389.
The apparatus of U.S. Pat. No. 4,835,389 has a tube, or light pipe, extending into a container; an internal reflectance element (IRE) at the bottom of the tube (supported by the tube); and a beamsplitter outside the tube which directs radiation from a source through the tube to the IRE, and directs radiation returning from the IRE to a detector.
As explained in U.S. Pat. No. 4,835,389, there are major benefits obtainable from deep immersion spectroscopy. One of the most important uses is in "batch process kettles", wherein liquid in a container is processed. Such processing usually involves chemical reactions, but it might also involve the non-reactive mixing of ingredients. In such processing kettles, the availability of in-situ, real-time spectroscopic analysis during the processing period can be of great practical value. It can provide information as to the progress of the processing, thereby permitting timely determination that the process has been completed. It can also provide valuable insights leading to possible improvements in future processing procedures.
Another major use of deep immersion spectroscopy relates to material in "storage drums". Because of the problem of material changes (deterioration) due to lengthy storage, it is desirable (and may be required by laws or regulations) to be able to promptly analyze the current condition of the material. Furthermore, in the case of hazardous or highly reactive materials, it is often desirable to positively identify a material (independent of its labeling) prior to using it in a process.
The apparatus of U.S. Pat. No. 4,835,389 may be referred to as a "colinear path" design, because a single ended IRE is used. Both the radiation entering the IRE and the radiation leaving the IRE pass through the upper end of the IRE; and both the entering and returning beams are in the same passage, or light pipe, between the beam splitter and the IRE.
An important objective of the present invention is to increase the signal transmission (radiation throughput) for a given path length and light pipe diameter.
The disclosure of the parent application Ser. No. 353,969 showed a dual light pipe structure, in which the entering and returning beams travel in separate, parallel light pipes. An IRE is located at the lower end of the structure. Its preferred location was stated to be in the lower end of one of the light pipes. And a separate retroreflector device was included to reverse the direction of the light. Certain claims were broad enough to cover an arrangement in which the IRE itself constitutes the direction-reversing means. However, such a structure was not illustrated.
Three prior art patents cited in the parent application disclose immersion structures in which a generally cone-shaped structure was mounted at the end of an analytical device intended to be immersed in a liquid sample. Michel et al U.S. Pat. No. 3,751,672 discloses an immersible refractometer, which includes parallel light pipes (105 and 106 in FIG. 10) and a "probe tip" 104 having internal reflecting surfaces whose angles of incidence are less than the critical angle. Such a device is not relevant to spectroscopic analysis, but rather is used to measure the index of refraction of the sample.
Two other prior art patents cited in the parent application - Schar et al U.S. Pat. No. 4,826,313 and McLachlan et al U.S. Pat. No. 4,829,186 - disclose immersible devices for use in ATR spectroscopy. Each of those patents has an ATR tip immersed in liquid ("conical sapphire 1" in Schar; and "trapezoidal prism 9" in McLachlan).
A fundamental difference between the analytical purposes of the present application and those of the Schar and McLachlan patents is the radiation wavelengths for which they are intended. Both Schar and McLachlan use optical fibers to conduct light toward and away from the ATR element. Although there is a substantial body of art relating to the use of fiber optic probes in spectroscopic systems, such art is not useful for the purpose of the present invention. Its inapplicability is due to the fact that fiber optic transmission characteristics are not adequate for use in analyzing mid-infrared (MIR) wavelengths. However, MIR is by far the most useful spectral region for studying the properties of the organic molecules fundamental to fields ranging from polymers to pharmaceuticals.
The inapplicability of optical fiber light guides to MIR wavelengths is acknowledged in this quotation from an article relating to fiber optics by Degrandpre and Burgess in Applied Spectroscopy (Vol. 44, No. 2, 1990, page 274):
"The mid-IR fibers suffer from high material absorption and scattering and poor mechanical and chemical stability. Therefore, they are not well suited for polymer-clad evanescent field sensors in their present stage of development."
For reasons which will be discussed in detail below, the objectives of the present invention require light pipes, not optical fibers, as the radiation transmitting (and guiding) structures. The term "light guide" is generic, including both "light pipes" and "optical fibers", but the two species are fundamentally different.
Optical fibers are solid materials having an index of refraction which is higher than the surrounding medium. The surrounding medium could be air, or it could be any material having a lower index of refraction than the optical fiber. There is an interface between a material of a higher index and a material of a lower index. The light propagates through the material of higher index, and is at a high enough incidence angle so that it is totally reflected at the interface. There is total internal reflection as the light channels down through the optical fiber.
In the case of a light pipe, the light pipe either is a metallic material, or a material that is internally coated with a metal that is a good reflector. The light travels through air, which is a medium having a low index of refraction, and is reflected off the metallic surfaces as it travels down through the light pipe.
The operation of a light pipe is essentially independent of radiation wave length, since the radiation is propagating through either air, dry nitrogen, vacuum, or the like, so the medium the light is propagating through is relatively transparent at all wavelengths. In contrast, an optical fiber light guide has its wavelength coverage limited by the transmission ability of the solid material that is used. The materials that are used for fiber optics have much better transmission in the visible and near-infrared than they do in the mid-infrared. Most of them become very highly absorbing in the mid-infrared.